Mobile terminal displays such as active matrix organic light emitting diode (AMOLED) often use low temperature poly-silicon thin film transistor (LTPS TFT) as a basic driving component in the driving circuit of array substrate. Generally, LTPS technology inherently makes TFT threshold voltages (Vth) discretely inconsistent. Because OLED pixel driving TFTs directly control driving current and brightness of OLED display, inconsistency in the threshold voltage may often cause undesired effect on image quality.
Using pixel circuit having Vth compensation function is often an effective way to improve the Vth discrete inconsistency. When an OLED driving TFT in such circuit is functioning, a driving signal written to the TFT gate electrode includes two components: a pixel OLED light emission brightness signal and a threshold voltage (Vth) compensation signal based on the driving TFT characteristics. This approach is similar to thin film transistor liquid crystal display (TFT-LCD), which generally includes two components in a driving signal and maintains the voltage level by a storage capacitor in a display frame period.
Alternatively, the pixel brightness signal is often generated by a driving integrated circuit (DrIC) and is written to the storage capacitor while the driving TFT Vth compensation signal is incrementally written to the storage capacitor by shorting between a gate electrode and a drain electrode of the driving TFT during a refreshing phase. Based on the two different means of supplying pixel brightness signal to the storage capacitor, pixel circuits are divided into two technical categories.